1. Field of the Invention
The present invention relates generally to the field of telephony. More specifically, the present invention discloses a codec for supporting PCM modem communications and conventional voice communications over a universal digital loop carrier.
2. Statement of the Problem
PCM Modem Communications. Various types of PCM modem communications have gained wide acceptance in recent years in response to the limited data transfer rates that are possible with analog modems. For example, the V.90 protocol adopted by the International Telecommunications Union (ITU) provides a specification for downstream modem data transfer rates up to 56 Kbps over the standard public switched telephone network (PSTN). It should be noted that the terms "public switched telephone network" or "PSTN" are used herein as these terms are commonly used in the United States to refer generally to the digital network between telephone company central offices. V.90, X2, K56flex, and other PCM modem systems overcome the limitations imposed on previous analog modems by exploiting the digital connection that most data service providers and internet service providers (ISPs) use at their end to connect to the PSTN.
The PSTN was designed for voice communications. By artificially limiting the sound spectrum to just those frequencies relevant to human speech understanding, the required bandwidth for each voice channel could be reduced, thereby increasing the number of voice channels that can be handled simultaneously when multiplexing is used. While this works well for voice communications, it imposes limits on data communications. In particular, existing V.34 modems that use conventional phase and amplitude modulation are optimized for the situation where both ends connect by analog lines to the PSTN, and are based on the assumption that both ends of the connection suffer impairment due to quantization noise introduced by analog-to-digital converters (ADCs).
The ADC samples the incoming analog waveform 8,000 times per second and outputs a pulse code modulation (PCM) code for each sample. The sampling system uses 256 discrete 8-bit PCM codes. The PCM codes sent across the PSTN can only approximate the original analog waveform because the sample values must be one of the 256 discrete codes. The amplitude difference between the original waveform and the reconstructed quantized waveform is called quantization noise.
Quantization noise and other impairments limit V.34 communications to about 35 Kbps. However, quantization noise affects only analog-to-digital conversion, not digital-to-analog conversion, which results in the principal advantage of the PCM modem protocols. FIG. 1 provides a diagram of a typical V.90 communications configuration. The high speed data direction for V.90 modems is from the central server modem 11 to the subscriber modem 14 (i.e., left to right in FIG. 1). The server modem 11 could be a gateway to an internet service provider (ISP) as this example shows. If there are no analog-to-digital conversions between the V.90 digital modem 11 and the PSTN 12, and since the V.90 digital modem uses only the 256 discrete PCM codes available on the digital portion of the phone network, then this exact digital information reaches the subscriber's CO line interface 13, and can be correctly decoded by the subscriber's analog modem 14, without information being lost in the conversion process. The encoding process in the server modem 11 uses only in most instances the 256 PCM levels (or a subset) used in the digital portion of the phone network. This significantly reduces the quantization noise that would be associated with an analog-to-digital conversion. The PCM codes are converted at the local telephone company's central office (CO) 13 to an analog signal that is sent to the subscriber's analog modem via a copper pair. The subscriber's analog modem 14 reconstructs the PCM codes from the analog signals it receives, decoding what the ISP 11 sent with no resulting loss of information.
Eight bit PCM codes are transmitted by ISP's V.90 digital modem 11 at the rate of the 8000 codes per second, so that the ISP modem's symbol rate is equal the phone network's sample rate. At the subscriber's end, the V.90 analog modem 14 must discriminate among the 256 analog voltages to recover the PCM codes (symbols). If this could be done at a rate of 8000 codes per second, the resulting download speed would be 64 Kbps (8000.times.8 bits per code). However, this is not often possible. Even though the quantization noise problem is removed, other noise sources exist. These arise principally from thermal noise, switching noise, and cross-talk. In addition, network DACs are not linear, but follow a non-linear conversion rule (.mu.-law in North America and A-law elsewhere) with very small step sizes near zero. These problems can make it impractical to use all 256 discrete codes, because the corresponding DAC output voltage levels near zero are too closely spaced to accurately represent data on an analog loop having a degree of noise. Therefore, the V.90 encoder uses various subsets of the 256 codes to eliminate detection errors between DAC output signals most susceptible to noise. Furthermore, the output of the DAC is band limited to about 3500 Hz by a reconstruction filter and connecting circuits that reduce the channel capacity (twice the available bandwidth) to approximately 7000 symbols per second. The limited bandwidth makes the symbols more difficult to distinguish from one another, a problem due to inter-symbol interference, or ISI. Therefore, the V.90 downstream modulator interleaves symbols periodically that contain lower information content, but serve to mitigate ISI.
The V.90 protocol is asymmetrical. As previously discussed, the V.90 protocol is capable of downstream data rates of 56 Kbps because little information is lost in the digital-to-analog conversion. However, upstream communications go through an analog-to-digital conversion in the telephone company central office line interface, which under V.90 does not have the ability to support PCM symbols and thus limits the channel to lower data rates. For this reason, V.90 modems limit upstream communications to V.34 speeds.
Digital Loop Carriers (DLC). Communications from the local-telephone central office (CO) to the subscriber's premises have traditionally been carried over a pair of copper wires using analog signals. This is commonly referred to as an "analog loop." In recent years, several types of situations have arisen that require extending digital communications beyond the central office. For example, where a group of subscribers 26-29 are geographically close to one another but relatively remote from the central office, the local exchange carrier may extend a digital carrier or a digital subscriber line (DSL) 25 from the central office to a remote terminal 24 in the subscriber's area, as illustrated in FIG. 2. Communications over the digital facility are handled using any of a number of standard digital protocols (e.g., T1 or T3). Analog loops are then extended from the remote terminal 24 to each of the subscribers 26-29 in the carrier service area. When dealing with a large number of subscribers (e.g., up to 1000 or more), this type of system is commonly known as a digital loop carrier or DLC. Smaller configurations are sometimes referred to as a "mini DLC."
The universal DLC (or UDLC) shown in FIG. 2 works well for voice communications and is satisfactory for data communications at V.34 speeds or lower. But, this type of DLC is incapable of V.90 communications due to the analog or "universal" interface between the central office switch 21 and the central office terminal 22 (COT) at the headend of the digital facility. As previously discussed, this analog interface requires an analog-to-digital conversion at the CO terminal 22 resulting in quantization noise. The analog interface is largely the result of the historical design for CO switches, that were based on directly providing analog loops to individual subscribers 23. Many newer CO switches are capable of providing a digital interface for a DLC, but many existing switches are not, and would require expensive retrofitting or replacement.
Subscriber Line Multiplexer. Another related situation arises when a subscriber wants to add a second or third telephone line to accommodate a fax or modem. The local exchange carrier has a limited number of wire pairs in each geographic area and installing new wires is often very expensive. To address this problem, a subscriber line multiplexer, universal digital carrier (UDC), or digital added main line, can be used to provide additional lines over a single pair of wires as depicted in FIG. 3. Here again, a digital facility, in this case a DSL 35, is extended from the central office to a remote terminal (RT) 34 typically located on the subscriber's premises. Several analog loops extend from the RT 34 for telephone 36, fax, or modem 37 connections. Communications for each of the analog loops are multiplexed over the DSL 35 using any of a number of standard digital communications protocols (e.g., 2B1Q). One type of subscriber line multiplexer is commercially available from Raychem Corporation under the name MINIPLEX.TM.. Here again, the problem is the analog interface between the CO switch 31 and the central office terminal 32 (COT) at the headend of the DSL 35. This analog interface requires an analog-to-digital conversation at the COT 32, resulting in quantization noise.
Due to their conceptual similarities, a subscriber line multiplexer may be considered to be a type of DLC. Any references throughout the remainder of this application to DLCs should be interpreted as including subscriber line multiplexers. The problem addressed here is often aggravated in the case of a subscriber line multiplexer since these systems are frequently installed to provide a second line for use with a modem.
A subscriber line multiplexer can be connected by an analog interface to a central office switch 31, as illustrated in FIG. 3. Alternatively, a subscriber line multiplexer can be used at the remote end of a DLC to provide an additional telephone line from the DLC remote terminal to a subscriber in the carrier service area. Here again, a subscriber line multiplexer could be connected by an analog interface to the DLC remote terminal. This would result in two analog-to-digital conversions in the downstream path. The first occurs at the CO terminal of the DLC, and the second occurs at the CO terminal of the subscriber line multiplexer.
Other Related Art. Other related art in the field includes the following:
______________________________________ Inventor Patent No. Issue Date ______________________________________ Ayanoglu et al. 5,394,437 Feb. 28, 1995 Ayanoglu et al. 5,528,625 June 18, 1996 ______________________________________
Ayanoglu et al., "An Equalizer Design Technique for the PCM Modem: A New Modem for the Digital Public Switched Network," IEEE Transactions on Communications, vol. 46, no. 6, pages 763-774 (June 1998).
U.S. Pat. No. 5,494,437 (Ayanoglu et al.) discloses a high-speed modem synchronized to a remote codec. The modem operates reliably at symbol rates up to twice its bandwidth when it is controlled by the clock of the receiving ADC.
U.S. Pat. No. 5,528,625 (Ayanoglu et al.) discloses a quantization-level-sampling (QLS) modem that includes means for separately equalizing each loop in an end-to-end digital telephone system network connection by employing a plurality of transmitter filters and a plurality of receiver filters in such a way that, in the upstream direction, the voltage samples seen by the codecs are equivalent to the network quantization levels transmitted by the modem, and in the downstream direction, the voltage samples seen by the modem are equivalent to the network quantization levels encoded by the codecs. These patents by Ayanoglu et al. relate to the fundamental principles employed in V.90 and other PCM modems and contribute to the understanding of the issues in the present disclosure.
The article by Ayanoglu et al. in IEEE Transactions on Communications discuss equalization techniques for use with PCM modems.
3. Solution to the Problem
The present invention solves the problem of providing PCM modem communications over a DLC having an analog interface to the central office. In particular, the present invention accomplishes this by employing a codec in the CO terminal that includes at least one adaptive equalizer and synchronizing the codec to the CO clock to minimize resampling error. The CO line interface and codec are typically housed within the same building at the central office and are located a relatively short distance from the CO switch. This results in an environment that is relatively noise-free and unchanging, so that adaptive training is only infrequently required. These features allow the codec to accurately recreate PCM codes at data rates sufficient to support V.90, X2, and K56flex communications, and also to support conventional voice communications and data communications. More generally, the present invention can be used whenever PCM coding is used in communications.